Professional Drainage Sizing — Fast and Accurate
Quick field sizing tools backed by NRCS, FHWA, and ASCE standards. Get accurate pipe sizes, drain capacities, and pond volumes you can use in bids and proposals.
Most-Used Contractor Tools
The calculators contractors reach for most often. All include shareable URLs and unit toggling.
Rational Method Calculator
Enter runoff coefficient, rainfall intensity, drainage area, and return period to calculate Rational Method peak discharge (Q = CiA), adjusted C, and a transparent calculation breakdown.
Open CalculatorManning's Equation for Pipe Flow Calculator
Enter circular pipe diameter, slope, roughness, and flow depth to calculate full- or partial-flow discharge, velocity, and hydraulic properties with Manning's equation.
Open CalculatorFrench Drain Calculator
Enter trench, aggregate, pipe, and soil parameters to calculate French drain storage, infiltration, and pipe capacity using Darcy's law and Manning's equation.
Open CalculatorCatch Basin Calculator
Enter design inflow and inlet geometry to estimate HEC-22 weir/orifice inlet capacity, then screen outlet-pipe capacity with Manning's equation and preliminary basin and sump dimensions.
Open CalculatorTrench Drain Calculator
Calculate trench drain capacity using Manning's equation.
Open CalculatorVegetated Swale Capacity Calculator
Calculate vegetated swale capacity, normal depth, velocity, freeboard, and Froude number with Manning's equation and vegetation-retardance roughness.
Open CalculatorDetention Pond Sizing Calculator
Estimate preliminary detention pond storage with the Modified Rational Method and size orifice or weir outlets from watershed and allowable-release criteria; use level-pool routing for final design.
Open CalculatorDownspout Sizing Calculator
Enter roof area and rainfall intensity to select round or rectangular downspout sizes from IPC sizing tables.
Open CalculatorOpen Channel Flow Calculator (Manning's Equation)
Calculate discharge, velocity, hydraulic properties, or normal depth with Manning's equation for rectangular, trapezoidal, triangular, parabolic, and surveyed station/elevation channel shapes.
Open CalculatorInlet Interception Calculator
Enter gutter flow and grate, curb-opening, or combination-inlet geometry to calculate HEC-22 interception efficiency, captured flow, and bypass flow.
Open CalculatorBuilt for the Field
Shareable URLs
Every calculation generates a URL you can share with clients or include in proposals.
Unit Toggle
Switch between imperial and metric units with one click. Results update instantly.
Standard Methods
Every calculator uses published methods from NRCS, FHWA, ASCE, and other authorities.
Reference Tables
Quick access to Manning's n, pipe sizes, runoff coefficients, and soil properties.
Quick Reference Tables
The values you look up most often, all in one place.
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